In active matrix-type liquid crystal displays, a twisted nematic (TN) liquid crystal material is used, for among other reasons, so that adequate response characteristics and contrast can be obtained under drive by a low voltage. There are two display modes of the TN-type liquid crystal display. One of the two display modes is a normally-white mode in which a bright display is obtained in a state with no application of voltage and the other is a normally-black mode in which a dark display is obtained in a state with no application of voltage. In the normally-black mode, coloring occurs in a dark state with no application of voltage (due to a condition of shielding light differently depending on the wavelength of the light). This is called optical rotatory dispersion. When it occurs, the contrast ratio will decrease. However, in the normally-white mode, if a sufficient voltage is applied, a good dark state can be obtained, and therefore a high contrast ratio can be implemented. It is for this reason, that a normally-white mode liquid crystal display is generally used.
Each of the above modes can be subdivided into two optical modes, depending on whether or not the transmission axis of a polarizing plate on the incidence side is set in parallel to the rubbing direction on the incidence side. As shown in FIG. 1(a), if a transmission axis 12 of a polarized plate on the incidence side is set in a direction parallel to a rubbing direction 14 on the incidence side, an extraordinary ray is transmitted through the liquid crystal material. Therefore, the mode in which such an extraordinary ray is transmitted is called extraordinary-ray dominant mode. As shown in FIG. 1(b), if the transmission axis 12 of the polarizing plate on the incidence side is set to make a right angle with the rubbing direction 14 on the incidence side, an ordinary ray is transmitted through the liquid crystal. Therefore, the mode in which such an ordinary ray is transmitted is called ordinary-ray dominant mode. A reference number 16 shown in FIG. 1(a) and FIG. 1(b) indicates the rubbing direction on the output side.
The viewing-angle dependence of the light transmittance and contrast ratio of the normally-white mode liquid crystal display with, for example, eight-level gray scale is compared for extraordinary-ray dominant mode (hereinafter called e mode) and ordinary-ray dominant mode (hereinafter called o mode). The gray scales are G7 and G0, in order, from the brightest level to the darkest level. Corresponding voltages V0 to V7 (for example, 0 to 5 volts) are applied, respectively.
FIG. 2(a) and FIG. 2(b) show viewing-angle dependence for relative light transmittance (hereinafter simply called transmittance) in the e mode. The graph of FIG. 2(a) shows viewing-angle dependence in the horizontal direction (.+-.50 degrees). The graph of FIG. 2(b) shows viewing-angle dependence in the vertical direction (.+-.50 degrees). In the specification, for the horizontal direction, the rightward and the leftward directions are the positive and the negative directions, respectively. For the vertical direction, the upward and the downward directions are the positive and negative directions, respectively. However, such definitions are arbitrary. In FIG. 2(a) and FIG. 2(b), for example, curves labeled as V0 show viewing-angle dependence of transmittance for voltage V0 in the horizontal and the vertical directions, respectively. In FIG. 3(a) and FIG. 3(b), similarly, viewing-angle dependence of transmittance in the o mode is shown.
As is apparent from FIG. 2(b), with respect to the viewing-angle characteristic of transmittance (T) in an up-viewing zone, in the e mode the curve indicated by V0 is below a curve indicated by V1 at angles greater than +25 degrees. Essentially, the application of voltage V0 should cause transmittance to be maximum. However, the application of voltage V0 does not cause the highest transmittance at angles in excess of +25 degrees in the up-viewing zone, but the application of voltage V1 or V2 causes higher transmittance than V0. Such a phenomenon, that is that a correspondence of the order of applied voltage levels with that of gray scale levels is lost, is hereinafter called, for convenience of explanation, "brightness inversion" or "gray-scale inversion". As is apparent from FIG. 3(b), in the o mode, the viewing-angle characteristic of transmittance in up- and down-viewing zones has a similar tendency. However, in comparing the o mode with the e mode, it is found that the former is excellent in its characteristics. In comparing the viewing-angle characteristic of transmittance in right- and left-viewing zones in the o mode with that in the e mode, both the o and the e modes are excellent in their characteristics, but the latter is a little better than the former. A range of viewing angles within which gray-scale inversion does not occur is indicated by circles along lines representative of transmittance of 100%. Transmittance is also referred to herein as gradation.
FIG. 4(a) and FIG. 4(b) show viewing-angle dependence of contrast ratios CR in e mode. A contrast ratio is defined as contrast with the darkest gray-scale level G0 corresponding to the highest applied voltage V7. The graph of FIG. 4(a) shows viewing-angle dependence in the horizontal direction (.+-.50 degrees). The graph of FIG. 4(b) shows viewing-angle dependence in the vertical direction (.+-.50 degrees). For example, in FIG. 4(a) a curve indicated by V0 shows viewing-angle dependence in the horizontal direction with respect to a ratio of transmittance (gray scale G0) corresponding to voltage V0 to transmittance (gray scale G7) corresponding to voltage V7. Similarly, a curve indicated by V6 shows viewing-angle dependence in the horizontal direction with respect to a ratio of transmittance (gray scale G6) corresponding to voltage V6 to transmittance (gray scale G7) corresponding to voltage V7. Further, in FIG. 4(b) a curve indicated by V0 shows a viewing-angle dependence in the vertical direction with respect to a ratio of transmittance (gray scale G0) corresponding to voltage V0 to transmittance (gray scale) G7) corresponding to voltage V7. Similarly, a curve indicated by V6 shows viewing-angle dependence in the vertical direction with respect to a ratio of transmittance (gray scale G6) corresponding to voltage V6 to transmittance (gray scale G7) corresponding to voltage V7.
FIG. 5(a) and FIG. 5(b) show viewing-angle dependence for contrast ratios CR in a o mode, in a similar manner.
As shown in FIG. 2(a) and FIG. 2(b), FIG. 3(a) and FIG. 3(b), FIG. 4(a) and FIG. 4(b) and FIG. 5(a) and FIG. 5(b), a range of viewing angles within which gray-scale inversion does not occur, is indicated by circles along the lines for the contrast ratio of 100. With respect to a contrast ratio, one at the brightest level is actually predominant over others. Therefore, in FIG. 6(a) and FIG. 6(b) a comparison of the ratios of transmittance (gray scale G0) corresponding to voltage V0, to transmittance (gray scale G7) corresponding to voltage V7 in o mode and in e mode, is made. As is evident from FIG. 6(a) and FIG. 6(b), the viewing-angle characteristic of contrast ratio in e mode is superior in the right- and left-viewing zones. With respect to up- and down-viewing zones, o mode is superior in the down-viewing zone and e mode is superior in the up-viewing zone.
Prior art relating to the present invention is Japanese Published Unexamined Patent Applications (PUPA) No. 61-121087, No. 62-196625, and No. 1-201622.
In the prior art, as described above, the viewing-angle characteristics for contrast ratio and gradation of a normally-white mode liquid crystal display have both advantages and disadvantages in both o or e mode. In both modes good contrast ratio and good gradation cannot be simultaneously obtained at a large viewing angle.